The present invention relates to wireless telecommunication systems, and more particularly to the use of antenna arrays in wireless telecommunication systems.
In a wireless telecommunication system that includes multiple mobile subscriber units and a base site connected to an antenna array that includes multiple antennas, a broadcast by the base site to an active subscriber unit can interfere with a broadcast to another active subscriber unit. The interference can cause unacceptable degradation of the radio frequency (RF) signal received by the subscriber unit, resulting in diminished performance and possibly a dropped call. In order to minimize the interference, methods have been developed for adaptively forming an antenna array beam pattern that results in a more focused broadcast of a signal to the intended subscriber unit and reduced interference with other active subscriber units using the same base site and antenna array.
Many techniques have been proposed for adaptively forming an antenna array beam pattern. A number of these techniques provide for focusing an antenna array beam in the direction of maximum received (by the base site from the subscriber unit) signal strength. That is, the techniques determine a separate amplitude and phase adjustment for each portion of a signal received from a subscriber unit via each of the multiple antennas before the signal portions are combined, thereby allowing the base site to resolve the received signal and interfering signals, nulling out the interfering signals and optimizing the received signal. When the base site transmits a signal to the subscriber unit, the amplitude and phase adjustments that are determined based on the received signal are in turn applied to each portion of the signal that is being transmitted by each of the multiple antennas. There may be situations, however, when it is disadvantageous to form an antenna beam for the purpose of broadcasting a signal to a subscriber unit. For example, a subscriber unit might be located behind an obstruction, with the result that a signal focused in the direction of maximum received signal strength, which is the perceived direction of the subscriber unit, is instead merely focused on an edge of the obstruction. A widely broadcast signal will more easily diffuse around the obstruction than a narrowly beamed signal and will have a better chance of reaching the subscriber unit than the beamed signal. Another situation where a focused beam is disadvantageous is when the subscriber unit is located in an environment where fading makes it exceedingly difficult to determine a location of the subscriber unit based on received signal strength. A beamed signal may end up being targeted in the wrong direction. Or a subscriber unit may be operating at a declining power level due to problems with the unit""s power source and will not be able to acceptably receive a slightly misdirected beamed signal. Therefore, a need exists for a method and apparatus that provides a determination of when a situation is appropriate for employment of an antenna array beamforming technique.
In a communication device that includes an antenna array including multiple antennas, the present invention provides a method and apparatus for determining whether to employ an antenna array beamforming technique. In one embodiment, the communication device determines a bearing from the communication device to a communication unit and determines a direction of arrival of a signal received from the communication unit by the communication device via each of the multiple antennas. The communication device compares the bearing to the direction of arrival, determines a difference between the two, and compares the determined difference to a predetermined difference threshold. The communication device then determines whether to employ an antenna array beamforming technique based on the comparison of the determined difference to the predetermined difference threshold.
In an alternative embodiment, the communication device determines a distance between the base site and the communication unit and compares the determined distance to a predetermined distance threshold. The communication device then determines whether to employ an antenna array beamforming technique based on the comparison of the determined distance to the predetermined distance threshold.
In another alternative embodiment, the communication device determines multiple signal quality metrics based on multiple signals received from the communication unit, compares the multiple signal quality metrics to each other, and determines whether to employ an antenna array beamforming technique based on the comparison of the multiple signal quality metrics.
By determining whether to employ an antenna array beamforming technique based on a comparison of the determined difference to the predetermined difference threshold, the determined distance to the predetermined distance threshold, or the multiple signal quality metrics to each other, the present invention provides for the employment of a maximum beamwidth broadcast in an environment where a misdirected beamed signal may not be acceptably received by the communication unit and for the employment of an antenna array beamforming technique in an environment where a beamed signal can be acceptably directed to the communication unit.